Photodiodes are used in a variety of applications such as biological and chemical analysis, as well as signal detection in fiber optic communication systems. Electrical noise is a well-documented natural phenomenon that has a variety of contributing independent and dependent sources. As is well known to those skilled in the art, the Signal to Noise Ratio (SNR) is an important determining factor in the quality of such a system. A SNR>>1 is very desirable.
Referring to FIG. 1, a photodiode amplification circuit 100 has a very large value of resistance (“R”) which for a given gain, produces a better SNR than that of a circuit with a smaller “R” and a subsequent gain stage that creates the same end to end gain. There are several reasons for this, such as the interaction of the photodiode impedances with the amplifier 102. Another notable reason is that of maximized signal amplification while minimizing the thermal noise that is amplified by the amplifier 102.
The Root Mean Square (RMS) thermal noise of the resistor “R” is calculated as:                √(4 kTR) in nano-volts rms per √(Hertz) where        k=Boltzman's constant=1.33×1023         T=Temperature in Kelvin        R=ohms of the gain resistor        
For room temperature, the equation is simply √((1.6×10E-20)×R)
Therefore, when R is increased by a factor G, the signal directly increases by G, but the thermal noise only increases by √(G). Since (R×G)>√(R×G), large values of G provide (R×G)>>√(R×G). If the SNR were viewed as SNR=(R)/√(R), then from a limiting standpoint, the SNR increases as R increases. Therefore, it is very desirable to have “R” as large as possible.
There are secondary reasons for having “R” large, and practical reasons to not have “R” be too large. This is due to additional phenomena dominating when “R” is increased by large multiples. When used in photodiode and electrometer preamplifiers, increasing “R” by factors of several multiples of the shunt impedance of the photodiode can make significant differences in the SNR quality of the system.
Doubling or tripling “R” may cause a respective double or triple change in the output range (or “swing”) of the amplifier. Furthermore, a photodiode produces a unidirectional current. Therefore, the signal may only have a single (unidirectional) polarity as it increases. In the circuit of FIG. 1, the amplifier is in an inverting configuration. The output of the amplifier will only go from 0 volts to a negative value. This is due to the photodiode only producing a current that is meaningful in a unipolar fashion. It is important to note that in this case, since the positive amplifier range is unused, one half of the amplifier signal range is wasted or unused.
Some photodiode amplification circuits have attempted to solve this issue by adding active feedback to dynamically change the Direct Current (“DC”) operating point of the input (where the DC component is not of interest) with large signals. Referring to FIG. 2, a feedback circuit implementation 200 uses the output of an amplifier 202 to feed back a corrective DC signal to bring the input and output of the amplifier 202 within an acceptable operating range for the amplifier 202. However, this type of circuit allows undesirable noise to be fed back into the input of the amplifier 202. While a certain amount of filtering can be added, not all of the noise can be filtered out. All types of feedback systems may have sufficient frequency response, i.e. bandwidth, to perform a closed loop feedback function. The noise filter cannot be set inside the control loop bandwidth, or the control loop will not be effective in biasing the input circuit. Accordingly, a need exists for a device, method, and system for improving the signal to noise ratio of a photodiode amplifier with standard power supply voltages.